High octane gasoline is required for modern gasoline engines. Benzene has a high octane number value and has been previously blended into gasoline. However, as benzene is phased out of gasoline for environmental reasons, it has become increasingly necessary to rearrange the structure of the hydrocarbons used in gasoline blending in order to achieve high octane ratings. Catalytic reforming and catalytic isomerization are two widely used processes for this upgrading.
A gasoline blending pool is usually derived from naphtha feedstocks and includes C4 and heavier hydrocarbons having boiling points of less than 205° C. (395° F.) at atmospheric pressure. This range of hydrocarbon includes C4-C9 paraffins, cycloparaffins and aromatics. Of particular interest have been the C5 and C6 normal paraffins which have relatively low octane numbers. The C4-C6 hydrocarbons have the greatest susceptibility of octane improvement by lead addition and were formerly upgraded in this manner. Octane improvement can also be obtained by catalytically isomerizing the paraffinic hydrocarbons to rearrange the structure of the paraffinic hydrocarbons into branch-chained paraffins or reforming to convert the C6 and heavier hydrocarbons to aromatic compounds. Normal C5 hydrocarbons are not readily converted into aromatics, therefore, the common practice has been to isomerize these lighter hydrocarbons into corresponding branch-chained isoparaffins. Although the non-cyclic C6 and heavier hydrocarbons can be upgraded into aromatics through dehydrocyclization, the conversion of C6's to aromatics creates higher density species and increases gas yields with both effects leading to a reduction in liquid volume yields. Therefore, it is preferable to charge the non-cyclic C6 paraffins to an isomerization unit to obtain C6 isoparaffin hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert normal C6 and lighter boiling hydrocarbons and reforming to convert C6 cycloparaffins and higher boiling hydrocarbons.
In the reforming processing, C6 cycloparaffins and other higher boiling cyclic hydrocarbons are converted to benzene and benzene derivatives. Since benzene and these derivatives have a relatively high octane value, the aromatization of these naphthenic hydrocarbons has been the preferred processing route. However, many countries are contemplating or have enacted legislation to restrict the benzene concentration of motor fuels. Therefore, processes are needed for reducing the benzene content of the gasoline pool while maintaining sufficient conversion to satisfy the octane requirements of modern engines.
Combination processes using isomerization and reforming to convert naphtha range feedstocks are well known. U.S. Pat. No. 4,457,832 uses reforming and isomerization in combination to upgrade a naphtha feedstock by first reforming the feedstock, separating a C5-C6 paraffin fraction from the reformate product, isomerizing the C5-C6 fraction to upgrade the octane number of these components and recovering a C5-C6 isomerate liquid which may be blended with the reformate product. U.S. Pat. Nos. 4,181,599 and 3,761,392 show a combination isomerization-reforming process where a full range naphtha boiling feedstock enters a first distillation zone which splits the feedstock into a lighter fraction that enters an isomerization zone and a heavier fraction that is charged as feed to a reforming zone. In both the '392 and '599 patents, reformate from one or more reforming zones undergoes additional separation and conversion, the separation including possible aromatics recovery, which results in additional C5-C6 hydrocarbons being charged to the isomerization zone.
The benzene contribution from the reformate portion of the gasoline pool can be decreased or eliminated by altering the operation of the reforming section. There are a variety of ways in which the operation of the reforming section may be altered to reduce the reformate benzene concentration. Changing the cut point of the naphtha feed split between the reforming and isomerization zones from 82 to 93° C. (180° to 200° F.) will remove benzene, cyclohexane and methylcyclopentane from the reformer feed. Benzene can alternately also be removed from the reformate product by splitting the reformate into a heavy fraction and a light fraction that contains the majority of the benzene. Practicing either method will put a large quantity of benzene into the feed to the isomerization zone.
The isomerization of paraffins is a reversible reaction which is limited by thermodynamic equilibrium. The basic types of catalyst systems that are used in effecting the reaction are a hydrochloric acid promoted aluminum chloride system and a supported aluminum chloride catalyst. Either catalyst is very reactive and can generate undesirable side reactions such as disproporationation and cracking. These side reactions not only decrease the product yield but can form olefinic fragments that combine with the catalyst and shorten its life. One commonly practiced method of controlling these undesired reactions has been to carry out the reaction in the presence of hydrogen. With the hydrogen that is normally present and the high reactivity of the catalyst, any benzene entering the isomerization zone is quickly hydrogenated. The hydrogenation of benzene in the isomerization zone increases the concentration of napthenic hydrocarbons in the isomerization zone.
It has been discovered that placing a hydrogenation reaction zone in front of an isomerization reaction zone but downstream of the feed driers required for the isomerization catalyst allows savings by reduction of equipment count and cost as well as a reduction in the amount of hydrogen required for the process. Placing the hydrogenation reaction zone downstream of the feed driers, allows the product condensers and receiver that would normally be required downstream of the hydrogenation reactor to be eliminated. Because the receiver has been eliminated, there is no hydrogen venting required. Hydrogen is a valuable commodity to refiners who are in need of ways to reduce hydrogen usage. Furthermore, in the present invention, low pressure feed driers may be used. Driers that are only operated at low pressures are less costly than high pressure driers and the cost of the many valves associated with the driers for the purposes of regenerating the drier sieves is reduced significantly for low pressure driers. Finally, additional utility savings are realized by the elimination of the condensing equipment normally required downstream of the hydrogenation reaction zone.